New or fully regenerated catalysts will have both MoO.sub.3 and Fe.sub.2 (MoO.sub.4).sub.3 phases uniformly throughout the catalyst. However, when these catalysts are employed industrially for the manufacture of formaldehyde via methanol oxidation, they preferentially lose the MoO.sub.3 component. This component is mainly lost from the catalyst placed at the top (inlet) and any "hot" spot regions of the catalytic reactor, leaving this catalyst with surface regions having the less efficient Fe.sub.2 (MoO.sub.4).sub.3 component. This decreases the efficiency, activity and selectivity of the catalyst, thereby requiring regeneration of the catalyst.
Regeneration methods prior to the present invention have had a number of significant disadvantages. Notably, regeneration by prior art methods required removal of the catalyst bed, regeneration in a separate vessel, and reloading of the catalyst bed. Such a sequence of steps was very expensive and slow.
The non-uniform compositions of the resulting spent catalysts also created difficulties in uniform regeneration. In addition to the molybdenum trioxide component losses noted above from the top and hot spot regions, the catalyst at the bottom of the reactor collects and becomes enriched in molybdenum trioxide component.
Thus, there is a need for new methods of regeneration of these catalysts. More specifically, there is a need for methods that redistribute the molybdenum trioxide component of the individual catalyst particles in order to increase the composition homogeneity while increasing average catalyst activity and formaldehyde selectivity.
The present invention fulfills this need by providing a new in situ method for redistributing the molybdenum trioxide component. Another objective of the present invention is to provide a method for obtaining more active catalysts, thereby resulting in reduced energy requirements because lower reactor temperatures will be needed to achieve complete methanol conversion. Yet another objective of the present invention is to increase the yield of formaldehyde from methanol oxidation, again resulting in reduced energy requirements of the process. These and other benefits of the present invention are described below.